World markets are taking a hit.

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Overnight, the pound fell to its lowest level since 1985 – the biggest one-day plunge ever. In Japan the Nikkei slumped by the greatest extent since the Fukushima earthquake. The country's finance minister, Taro Aso, said he is "very concerned about the world economy", The Guardian reported. European markets all dropped when they opened: Germany's DAX was is currently down 6.72%; France's CAC down 8.37%, and Spain's IBEX down 10.65%.

Sinn Féin said the British government has "forfeited its mandate" to represent Northern Ireland.

The party's national chair, Declan Kearney, said there was a question over whether Northern Ireland, which voted 56% to 44% to stay in the EU, would remain part of the UK.

"English voters are dragging Northern Ireland out of the EU," he said in a statement carried by ITV News. "This British government has forfeited any mandate to represent the economic or political interests of people in Northern Ireland."

Senior EU government officials told BuzzFeed News they wanted to send a signal that no new concessions for the UK were forthcoming.

Donald Tusk, president of the European Council, acknowledge huge disappointment at the outcome of the referendum. But he added that the 27 other member states were "determined to keep our unity".

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Tusk said:

There's no hiding the fact that we wanted a different outcome of yesterday's referendum. I am fully aware of how serious, or even dramatic, this moment is politically. And there's no way of predicting all the political consequences of this event, especially for the UK. It is a historic moment but for sure not a moment for hysterical reactions. I want to reassure everyone that we are prepared also for this negative scenario. As you know the EU is not only a fair-weather project.

Over the past two days I have spoken to all the EU leaders, I mean Prime Ministers and Presidents as well as heads of the EU institutions, about the possibility of a Brexit. Today, on behalf of the twenty seven leaders I can say that we are determined to keep our unity as twenty seven. For all of us, the Union is the framework for our common future. I would also like to reassure you that there will be no legal vacuum. Until the United Kingdom formally leaves the European Union, EU law will continue to apply to and within the UK. And by this I mean rights and obligations.

All the procedures for the withdrawal of the UK from the EU are clear and set out in the Treaties. In order to discuss the details of further proceedings, I have offered the leaders an informal meeting of the twenty seven in the margins of the European Council summit. And I will also propose to the leaders that we start a wider reflection on the future of our Union.

Finally, it's true that the past years have been the most difficult ones in the history of our Union. But I always remember what my father used to tell me: What doesn't kill you, makes you stronger.

German foreign minister Frank-Walter Steinmeier said the Brexit result was "really sobering".

Martin Schulz, president of the European parliament, was quoted by BBC News as saying: "We respect the result. Now is the time for us to behave seriously and responsibly. David Cameron has his responsibilities for his country, we have our responsibilities for the future of the EU. You can see what is happening to sterling on the markets. I don't want the same thing to happen to the euro."

In a statement Turkey's EU minister Omer Celik said the country "respected" Britain's decision, but that it was "bad for Europe.

His full statement said:

"Brexit marks the beginning of a new period when important developments will take place regarding Europe's future. We monitor the situation closely," the statement said.

"Moving forward, there will be questions about Scottish independence, the situation in Gibraltar and joint security.

"Young people's support for the Remain campaign indicates that the idea of Europe has been successful.

"David Cameron failed to resist the extreme right's attacks against Turkey. When mainstream politicians can't act with common sense, they are drawn into the extreme-right agenda.

"Turkey has been a European state throughout history and Turkey not being an EU member doesn't change this fact.

"European values are humanity's shared values, on which, Turkey believes, the union must be based. But there have been serious problems with the implementation of European ideas in recent years. Isolationism would destroy European values.

"Turkey worked with the European Union to address the refugee crisis,one of the greatest challenges Europe faces today. What Europe needsis a fresh start including an update to existing mechanisms."

Australian prime minister Malcolm Turnbull, who's facing a general election, tried to reassure people there was no cause for alarm.

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"It is important to remember that the Australian economy is strong and resilient, and has weathered global shocks before and weathered them well," he said.

"I have no doubt that in due course, the British government will negotiate a satisfactory departure from the European Union. This could take several years, and in the meantime, I have no doubt that our relations with the United Kingdom, which are as close as any two nations' relations could be, will continue as positively and intimately as ever."

Barack Obama, who said Britain would be "at the back of the queue" for a trade deal if it voted for Brexit, said the "special relationship" between the US and UK is "enduring".

"The people of the United Kingdom have spoken, and we respect their decision. The special relationship between the United States and the United Kingdom is enduring, and the United Kingdom’s membership in NATO remains a vital cornerstone of US foreign, security, and economic policy," Obama said.

"So too is our relationship with the European Union, which has done so much to promote stability, stimulate economic growth, and foster the spread of democratic values and ideals across the continent and beyond. The United Kingdom and the European Union will remain indispensable partners of the United States even as they begin negotiating their ongoing relationship to ensure continued stability, security, and prosperity for Europe, Great Britain and Northern Ireland, and the world."

Donald Trump, who is coincidentally in Scotland for the reopening of his golf course, told reporters Brexit was a "great thing, I think it's going to be great".

"The people of the United Kingdom have exercised the sacred right of all free peoples. They have declared their independence from the European Union and have voted to reassert control over their own politics, borders and economy. A Trump Administration pledges to strengthen our ties with a free and independent Britain, deepening our bonds in commerce, culture and mutual defence. The whole world is more peaceful and stable when our two countries – and our two peoples – are united together, as they will be under a Trump Administration.

"Come November, the American people will have the chance to re-declare their independence. Americans will have a chance to vote for trade, immigration and foreign policies that put our citizens first. They will have the chance to reject today’s rule by the global elite, and to embrace real change that delivers a government of, by and for the people. I hope America is watching, it will soon be time to believe in America again."